The return of kwin_gles

Back in 4.x we provided two binaries for KWin: one compiled against OpenGL (kwin) and one compiled against OpenGL ES (kwin_gles). The reason for that is that one can only reasonably link either OpenGL or OpenGL ES and OpenGL ES is only a subset of OpenGL, so one needs to hide the OpenGL calls (especially the OpenGL 1 calls).

With 5.x we were no longer able to provide these two binaries. The reason for that is that OpenGL got “upgraded” in Qt and QtGui itself links either OpenGL or OpenGL ES. To keep the system’s sanity we decided to follow how Qt is compiled. If Qt is compiled with OpenGL support KWin gets compiled with OpenGL support, otherwise with OpenGL ES support.

That’s of course a reasonable design, but it means that it becomes difficult to test the OpenGL ES code paths. One needs a dedicated Qt and all other dependencies compiled against OpenGL ES. Or one needs a nice device like a Nexus 5 with Plasma mobile. This had resulted in breakage already as we were not able to test enough. Such times belong to the past as I have a nice Plasma mobile device to compile and test my KWin on.

But since we introduced OpenGL ES support through a compile time switch, many things have changed. KWin dropped the OpenGL 1 support which means that most of the code which wouldn’t compile with OpenGL ES is just gone. Furthermore we switched to use libepoxy, so we don’t link OpenGL or OpenGL ES at all, but libepoxy which does the right thing for us. With that we are able to remove all the compile time checks. Of course we need runtime checks to ensure that we don’t call functionality which isn’t available on OpenGL ES.

Now with the upcoming 5.5 release we are able to have one binary which serves both OpenGL and OpenGL ES. Note to distributions: the artifact kwinglesutils.so is no longer compiled, please adjust your packaging rules. KWin will use either OpenGL or OpenGL ES depending on what Qt uses.

Given that nowadays it’s also possible to create both an OpenGL and OpenGL ES context at runtime we can also make use of that and introduced a new value for our KWIN_COMPOSE environment variable: O2ES. If that’s specified KWin will use the EGL backend and create an OpenGL ES context. Although it in general is also possible to create an OpenGL ES context through glx we do not support that for simplification. As proof debug output (qdbus org.kde.KWin /KWin supportInformation) from my running KWin instance:

<code>Compositing
===========
Compositing is active
Compositing Type: OpenGL ES 2.0
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) Ivybridge Desktop 
OpenGL version string: OpenGL ES 3.0 Mesa 10.6.8
OpenGL platform interface: EGL
OpenGL shading language version string: OpenGL ES GLSL ES 3.00
Driver: Intel
GPU class: IvyBridge
OpenGL version: 3.0
GLSL version: 3.0
Mesa version: 10.6.8
X server version: 1.17.3
Linux kernel version: 4.2
Direct rendering: Requires strict binding: no
GLSL shaders:  yes
Texture NPOT support:  yes
Virtual Machine:  no
OpenGL 2 Shaders are used
Painting blocks for vertical retrace:  yes
</code>

So in a way we have kwin_gles back, it’s different as it’s no longer a dedicated binary, but it’s runtime switchable. For the moment the only way will be the environment variable. I’m reluctant to add a config option as that sounds like quite some chance for breakage.